1. Field of the Invention
The present invention relates to an electrical fuse device based on a phase-change memory element and to a corresponding programming method, in particular for a read-only memory (ROM) of the one-time-programmable (OTP) type, to which the following description will make reference, without this implying any loss in generality.
2. Description of the Related Art
As is known, in the manufacturing process of integrated circuits, one-time-programmable ROMs find a wide range of applications for permanent storage of information, or for forming permanent connections within integrated circuits. For example, these memories can be used for programming redundant elements in order to replace identical elements that have proven faulty during an electrical testing (operation known as EWS—Electrical Wafer Sorting), prior to carrying out packaging or soldering of the integrated circuits on the board, or else for storage of basic information regarding the integrated circuit, such as identifier codes or calibration information. In particular, the aforesaid information must be stored in a permanent way in order to be recovered after the packaging or soldering operations.
In order to produce the aforesaid memories using semiconductor technology, the use of E2PROM (Electrically Erasable Programmable Read-Only Memory) devices, fuse devices and anti-fuse devices has been proposed. However, for reasons that will be briefly set forth, the solutions referred to have some problems that do not make their use totally satisfactory within modern integrated devices.
In particular, E2PROM devices require oxide layers having a large thickness (for example, 7 nm) to prevent high leakage currents and sustain a charge stored on a corresponding floating terminal. The scales of integration required by modern integrated circuits do not always enable use of such large oxide thicknesses. Furthermore, the use of E2PROM devices in any case involves a high area occupation.
The fuse devices commonly used for the applications referred to above are programmed using a laser, which is used to cut a connection after the fuse device has been manufactured. Laser programming entails an additional process step, extraneous to semiconductor technology, and moreover calls for a perfect alignment of the laser with respect to the fuse device to be programmed.
Anti-fuse devices are typically based on the perforation of metal-insulator-metal structures to obtain low-resistance paths. Said devices require high programming voltages, and consequently involve high breaking voltages of the programming circuits associated thereto. Furthermore, said devices are generally of a horizontal type and involve a high area occupation.
Other types of semiconductor fuse devices that can be electrically altered, for example based on polysilicon resistors, have been proposed, for example in the U.S. Pat. No. 6,337,507 and in the patent application No. US 2003/0218492. However, none of said devices is optimized in terms of costs, manufacturing times, and programming times (which should be as short as possible).
Phase-change memories (PCMs) are moreover known, which exploit, for storage of information, the characteristics of materials that have the property of switching between phases having different electrical characteristics. For example, said materials can switch between a disorderly, amorphous phase and an orderly, crystalline or polycrystalline phase, and the two phases are associated to resistivities having considerably different values, and consequently to different values of a stored datum. Currently, the elements of Group VI of the periodic table, such as tellurium (Te), selenium (Se), or antimony (Sb), referred to as chalcogenides or chalcogenic materials, may advantageously be used to obtain phase-change memory cells. The currently most promising chalcogenide is formed by an alloy of Ge, Sb and Te, generically referred to as GST (for example, Ge2Sb2Te5).
The phase changes are obtained by locally increasing the temperature of the cells of chalcogenic material by means of resistive electrodes (generally known as heaters) set in contact with the region of chalcogenic material. A selection device (for example, a MOSFET or a bipolar transistor), is connected to the heater and is configured to enable passage of a programming electrical current through the heater. Said electrical current, by the Joule effect, generates the temperatures necessary for phase change. In particular, since the minimization of the area of contact between the heater and the region of chalcogenic material is a primary requisite in such devices, in order to ensure repeatability of the programming operations, the heaters generally have sublithographic sections (i.e., dimensions smaller than the dimensions that can be achieved with current lithographic techniques, for example smaller than 100 nm, down to approximately 5-20 nm).
A wide range of manufacturing processes have been proposed to obtain phase-change memory cells, and the configurations of the resulting memory cells are different, in particular as regards coupling between the heater and a corresponding chalcogenic region. For example, a microtrench architecture is described in U.S. Pat. No. 6,891,747, while a lance-shaped or ring-shaped tubular architecture is described in U.S. patent application Ser. No. 11/398,858, filed on Apr. 6, 2006.
Although advantageous as regards performance and manufacturing costs, PCMs cannot be used in the applications described above. In fact, the high temperatures that are generated during the processes of packaging or soldering on the board can lead to the change of state of previously programmed memory cells and the consequent loss of the stored information. In particular, the possibility exists that memory cells in the amorphous state will switch to the crystalline state on account of said high temperatures.